Newfound Virus is a Giant with Lifelike Properties

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Scientists have found the ocean's largest virus. Fortunately,
this microbial monster is a menace only to a particular
single-celled organism.

Based on the size of its genome, or complete DNA sequence, the
microbe dubbed CroV is the second to be considered a "giant
virus." The only virus with a larger genome lives in fresh water.

CroV's enormous, and surprising, genetic code further blurs the
boundary between viruses and cellular life, according to the
researchers who described it.

CroV is equipped with genes that allow it to repair its
genome, synthesize sugars and even gain more control over the
machinery that it hijacks within the host cells to replicate
itself.

"They take
over the cell, and they basically run the cell," said
Matthias Fischer, who described CroV for his doctoral
dissertation at the University of British Columbia. He added that
the production of new CroV viruses within an infected cell
resembles an assembly line.

Viruses are essentially genetic material wrapped in a thin
protein coat, and they must use the goods of a host in order to
make more of themselves. Traditionally, viruses were considered
nonliving. However, these discoveries about CroV add more weight
to the argument that viruses are alive, Fischer said.

Fischer found that CroV's genome contains approximately 730,000
base pairs, the building blocks of DNA. By comparison, the
largest virus on record, Mimivirus,
has a genome of about 1.2 million base pairs. Prior to
confirmation in 2003 that Mimivirus was indeed a virus, the
largest known virus had a genome of around 331,000 base pairs,
according to Fischer.

Despite its size, CroV is a threat only to the relatively small.
It infects a common, single-celled grazing creature called
Cafeteria roenbergensis. In fact, the virus is
approximately a twentieth the size of its host. (For a person who
stood 5-foot-6, or 1.7 meters, this would translate roughly into
being infected
by a virus the size of a softball.)

Not surprisingly, the infection kills Cafeteria
roenbergensis, according to Curtis Suttle, also a University
of British Columbia researcher who worked on the study. This tiny
creature may be the most abundant eukaryote, or complex celled
organism, in the ocean and perhaps the world, he said. This
category includes all animals, plants and many other organisms.

Mimivirus, meanwhile, lives in fresh water and infects amoebas,
which, like C. roenbergensis, are single-celled
creatures.

Hodgepodge of genes

C. roenbergensis's diet of bacteria and viruses
may explain the strange collection of genes possessed by the
giant virus that infects it. Perhaps the oddest of these include
the genes that code for the entire pathway to create a key
component to a bacterial outer membrane

"Who knows why that is in there?" said Suttle."As far as we know,
it (CroV) doesn't interact with bacteria at all."

CroV may have acquired these genes by picking up DNA from the
remains of a bacterium eaten by a cell the virus later infected,
according to the researchers. Something similar also may have
occurred with Mimivirus, which infects a bacteria-eating amoeba
and also appears to contain genes of bacterial origin. This is a
possible explanation for the origin of 10 to 20 percent of the
giant virus genes.

Other genes within CroV are even more mysterious. The researchers
could not recognize 51 percent of the genes they encountered in
the new virus. This is actually a low proportion – about 90
percent of the genes within certain viruses are unknown, Fischer
said.

"Every virus you pull out has a new set of genes that is unique
to this virus, that has never been seen before," he said.

This makes it unlikely that many viral genes have cellular
origins. It is currently hypothesized that viral
genes are ancient, and have never been part of cellular
organisms, he said.

The research is published today (Oct. 25) in the online early
edition of the Proceedings of the National Academy of Sciences.